The salt of .gamma.-(piperidyl)butyric acid (2) can be prepared by hydrogenating a salt of .gamma.-(2-pyridyl)butyric acid with an acid for conversion of the pyridine ring to a piperidine ring. It is known to prepare the salt of .gamma.-(piperidyl)butyric acid (2) by hydrogenating a salt of (.gamma.-pyridyl)butyric acid with an acid using a platinum oxide as a catalyst [J. Am. Chem. Soc., 69,2461 (1947)].
However, the above-mentioned conventional process hydrogenates the salt of .gamma.-(2-pyridyl)butyric acid with an acid at a concentration of 2% in the reaction mixture, namely a much diluted salt, consequently leading to a low reactor efficiency and a low productivity. Thus the process is not practical.
The above-mentioned process poses a further problem of necessitating a large amount of platinum oxide as the catalyst. The foregoing publication teaches the use of platinum oxide in an amount of as large as 4.3% by weight in terms of Pt based on a .gamma.-(2-pyridyl)butyric acid hydrochloride.
No investigation has been heretofore made about the use of other catalysts than platinum oxide for the hydrogenation of a salt of .gamma.-(pyridyl)butyric acid with an acid. It is an object of the present invention to provide a catalyst capable of hydrogenating a salt of .gamma.-(pyridyl)butyric acid with an acid to efficiently convert the pyridine ring to a piperidine ring, providing a process for preparing a salt of .gamma.-(piperidyl)butyric acid (2) with a high productivity.